Although, presently, lithium ion batteries are widely used as secondary batteries, studies have been conducted on sodium-ion secondary batteries in which sodium ion is employed in place of lithium ion of the lithium ion batteries.
The sodium ion batteries usually comprise an anode, a cathode and an electrolyte composition containing one or more solvent, and one or more conductive salt and/or additive.
International patent application WO 2010/109889 A1 discloses a sodium ion secondary battery which is provided with a positive electrode, a negative electrode having a negative electrode active material, and a nonaqueous electrolyte solution containing a nonaqueous solvent, wherein a hard carbon is used as the negative electrode active material and the nonaqueous solvent is substantially composed of propylene carbonate, a mixed solvent of ethylene carbonate and diethyl carbonate, or a mixed solvent of ethylene carbonate and propylene carbonate.
International patent application WO 2012/132813 A1 further discloses an additive for sodium ion secondary batteries containing a compound that is composed of a saturated cyclic carbonate having a fluoro group and/or a chain carbonate having a fluoro group, and a sodium ion secondary battery comprising a nonaqueous electrolyte solution which contains the additive for sodium ion secondary batteries and a nonaqueous solvent that is composed of a saturated cyclic carbonate or a nonaqueous solvent that is composed of a saturated cyclic carbonate and a chain carbonate; a positive electrode; and a negative electrode which has a coating film on the surface, said coating film being composed of a composite material containing carbon, oxygen, fluorine and sodium, and which contains a negative electrode active material that is composed of hard carbon.